Pulsewidth modulation apparatus, printer, and control method thereof

ABSTRACT

When pulsewidth modulation is performed with a digital data pattern, appropriate pulsewidth modulation is performed in accordance with variations of a device or a driving system. In a case of inputting data where one pixel is expressed by, e.g., 4 bits (16 tones), a 32-bit pattern having twice as many number of bits as a 16-bit pattern is prepared as a pattern corresponding to input data, and stored in digital data output unit  1001.  Pulsewidth addition circuit  1007  is provided for adding a fine-adjustment pulsewidth so as to achieve a target amount of light. The pulsewidth addition circuit  1007  is set so as to achieve a minimum pulsewidth that can generate a laser beam when input data is 0.

FIELD OF THE INVENTION

[0001] The present invention relates to a pulsewidth modulationapparatus, applied to an electrophotography system utilizing exposuremeans, such as a laser beam or the like, an image printing apparatususing the pulsewidth modulation apparatus, and a control method thereof.

BACKGROUND OF THE INVENTION

[0002] A construction of a general laser-beam printer is shown in FIG.11, and operation thereof is described.

[0003] A laser chip 23 comprises a laser diode and a photodiode whichreceives backlight of the laser diode. An LD driver 24 supplies adriving current Id for controlling emission of the laser diode. Amonitor current Im, detecting the amount of light emitted from thephotodiode, is inputted to the LD driver 24 to perform automatic powercontrol (APC) of the amount of light emission of the laser diode.

[0004] A modulated laser beam generated by the laser chip 23 ispolarized by a polygon mirror 18, which is fixed to a motor axle androtated in the direction of the arrow shown in FIG. 11, and scanned on aphotosensitive drum 20. An f-θ lens 19 is provided to condense thepolarized modulated laser beam on the photosensitive drum 20 at aregular linear velocity. The photosensitive drum 20 and printing tonerare electrostatically charged to predetermined levels in advance. Sincethe amount of toner attachment changes in accordance with the amount oflight irradiated on the photosensitive drum 20, a halftone image can beprinted. A BD mirror 21 is provided with a fixed mechanical positionrelation with the photosensitive drum 20. A laser beam reflected by theBD mirror 21 is inputted to a photoreceptive diode 22, and used todetect a data write starting position on the photosensitive drum 20. Anoutput of the photoreceptive diode 22 is inputted to a horizontalsynchronization signal generator 27 to generate a horizontalsynchronization signal BD. The signal BD is inputted to a pixelmodulator 25. The pixel modulator 25 generates a pixel clocksynchronizing with the horizontal synchronization signal BD or acoefficient-fold clock of the signal BD. Based on the pixel clock, aread clock RK for reading pixel data is inputted to a pixel datagenerator 26. The pixel data generator 26 outputs pixel data D and awrite clock WK to the pixel modulator 25. Based on the inputted pixeldata, a pixel modulation signal ON, which enables modulation of adesired amount of a laser beam, is outputted to the LD driver 24.

[0005] Furthermore, the above-described construction may be provided forfour photosensitive drums 1020 a to 1020 d as shown in FIG. 12. Byproviding each of the photosensitive drums with the structures forlaser-scanning and developers respectively having Y, M, C and Bk tonerlocated next to each other, it is possible to print a color image,produced by superimposing each of the color components, on a sheet ofprint paper 1028.

[0006] In order to express density of a print image (in a case of acolor image, density of each color component), the amount of lightcorresponding to the density of an image to be printed is irradiated asdescribed above.

[0007] In general, a pulsewidth modulation (PWM) system is known aseffective means to control the amount of light irradiation. FIG. 13shows an example of a construction which realizes a PWM system.

[0008] In FIG. 13, reference numeral 1 denotes a digital data outputunit which converts printing data (multivalued image data) from anexternal device, such as a computer or the like, to a multivalued datastring for each scanning line. Reference numeral 2 denotes a lookuptable which inputs the multivalued data outputted from the digital dataoutput unit 1, executes table conversion corresponding to predeterminedfunction processing, and outputs multivalued data. Reference numeral 3denotes a D/A converter which inputs the multivalued data from thelookup table 2, and outputs a corresponding analog voltage signal.

[0009] Reference numeral 5 denotes a horizontal synchronization signalgenerator based on the signal BD mentioned above; 6, a referencefrequency generator; 7, a timing signal generator which generates atiming signal for each component of the apparatus; and 8, a triangularwave generator which generates an analog triangular wave in accordancewith the timing signal (clock signal) supplied by the timing signalgenerator 7.

[0010] Reference numeral 4 denotes a comparator which compares theanalog signal based on image data (output signal of the D/A converter 3)with the triangular wave outputted by the triangular wave generator 7,and outputs the comparison result as a logical high/low pulse signal.Accordingly, the comparator 4 outputs a signal having a pulsewidthcorresponding to a pixel value. Reference numeral 8 denotes a rasterscanning print engine, having a construction for driving a laser devicein accordance with a pulsewidth of a pulsewidth modulation signal asdescribed above with reference to FIG. 11. Therefore, in one pixelcycle, light emission time of a laser beam is determined based on apixel value. In this manner, the amount of light emission can becontrolled.

[0011] The above-described construction realizes an image modulator, inwhich inputted multivalued image data is subjected to γ conversion withan appropriate function and subjected to pulsewidth modulation with aweight corresponding to the result of the γ conversion.

[0012] Note as the aforementioned comparator, although an analogcomparator is employed, by employing a digital comparator and generatinga digital triangular wave, a digital pulsewidth modulator which directlycompares the γ-converted multivalued image data with the digitaltriangular wave can be constructed. In the case of employing suchdigital system, characteristic variations due to a temperature drift andvariations of components, which are problematic in the analog method,can generally be made small.

[0013] However, in the case of employing the above-described digitalpulsewidth modulator which adopts a triangular wave comparison method,there are disadvantages, such as a necessity of a fairly high-speedcounter and a high-speed comparator, as well as an enlarged size of thecircuit.

[0014] In view of this, recently proposed is an image modulator in whichan output pattern of a pixel is expressed in advance by a string ofbinary data representing light emission/non light emission, plural typesof the output patterns are set in advance in a digital circuit for thenumber corresponding to values of inputted image tones, then uponinputting multivalued image data, the output pattern is selected andread out, and the read binary data strings are serially outputted.

[0015] For instance, assuming a case of inputting 4-bit input data (16tones), 16 bits are prepared as pattern data. For input data “3”, apattern having “1” for the first three bits and “0” for the subsequentthirteen bits is stored. When this data (value 3) is inputted,1110000000000000B (B indicates binary data) is sequentially outputted,thereby generating a signal having a corresponding pulsewidth.

[0016] However, the above technique has the following problems.

[0017] Because the number of bits in a light emission pattern is only2^(n) (or (2^(n))−1) while the number of bits of input data is n, thelight emission pattern is determined practically on a one-to-one basiswith the inputted image data. Therefore, it is impossible to correctvariations in input-output functions, due to the output driving patterncorresponding to each input data and unevenness in electric and opticalcharacteristics of a laser driver and laser device that follow theoutput driving pattern.

[0018] In a case where a block in which the output light emissionpattern is set is fixedly constructed with logical hardware, because thelight emission pattern is determined on a one-to-one basis with inputtedimage data, it is impossible to correct variations in input-outputfunctions, due to the output driving pattern corresponding to each inputdata and unevenness in electric and optical characteristics of a laserdriver and laser device that follow the output driving pattern.

[0019] If the aforementioned variations are individually measured inadvance in a stage of apparatus assembling adjustment and stored in ROMconstituting the block where the output light emission pattern is set,the variations of initial characteristics can be corrected individually.However, variations caused by environmental changes or variationsgenerated over time cannot be corrected. In a case of adopting a RAMsystem, values are initialized from the system side when starting up theapparatus. However, since this is only to rewrite set data held by thesystem, the problem is not solved.

SUMMARY OF THE INVENTION

[0020] The present invention has been proposed in view of theconventional problems, and has as its object to provide a pulsewidthmodulation apparatus, which enables appropriate pulsewidth modulation inaccordance with variations of a device or a driving system in a casewhere pulsewidth modulation is performed with digital data patterns, andto provide a printer utilizing the pulsewidth modulation apparatus, anda control method thereof.

[0021] In order to solve the above problems, the pulsewidth modulationapparatus according to the present invention has, for instance, thefollowing configuration. More specifically, a pulsewidth modulationapparatus for inputting multivalued pixel data expressed in a pluralityof bits, generating a signal having a pulsewidth corresponding to avalue of the multivalued pixel data, and outputting the generated signalto a predetermined print engine, comprises storage means arranged tostore a digital pattern in an address position corresponding to thevalue of each multivalued pixel data in order to generate abinary-valued pattern corresponding to the value of the multivaluedpixel data; and output means arranged to access the storage means byusing inputted multivalued pixel data as an address to read acorresponding binary-valued pattern, and output a pulsewidth signalcorresponding to a state of bits of the pattern, wherein the patternstored in the storage means has a number of bits larger than a numberthat can be expressed by a number of bits of the inputted multivaluedpixel data, and the pattern is rewritable.

[0022] Other features and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

[0024]FIG. 1 is a block diagram showing a main unit of a printeraccording to an embodiment of the present invention;

[0025]FIG. 2 is a block diagram of a light amount detector according tothe embodiment of the present invention;

[0026]FIG. 3 is a timing chart employed in the construction shown inFIG. 2;

[0027]FIGS. 4A and 4B are views showing a relation between a digitalpulsewidth and an additional pulsewidth of laser driving devices A andB;

[0028]FIG. 5 is a graph showing a bare characteristic of a relationbetween the pulsewidth and amount of light of the laser driving devicesA and B, and a characteristic upon correction performed for the amountof light 0;

[0029]FIG. 6 is a flowchart showing determination processing steps of apulsewidth addition amount for the amount of light 0 in FIG. 5;

[0030]FIG. 7 is a table showing a relation between the target amount oflight for each input data and a PWM width of each device according tothe embodiment;

[0031]FIG. 8 is a flowchart showing steps of PWM determinationprocessing;

[0032]FIG. 9 is a flowchart showing steps of PWM determinationprocessing;

[0033]FIG. 10 is a graph in which input data according to the presentembodiment and the amount of light obtained as a result of corrected PWMare plotted;

[0034]FIG. 11 is a block diagram showing a construction of an engineunit of a laser beam printer;

[0035]FIG. 12 is a view showing a construction of a color printer usingfour drums;

[0036]FIG. 13 is a block diagram showing a construction of an apparatusemploying analog PWM; and

[0037]FIG. 14 is a block diagram showing a construction of a two-beamsystem printer engine unit adopted by the present embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Preferred embodiments of the present invention will now bedescribed in detail in accordance with the accompanying drawings. Notein this embodiment, a laser beam printer is described.

[0039] A laser beam printer adopts a pulsewidth modulation (PWM) systemfor printing tones of an image. Known means to realize PWM is to converta multivalued image signal to an analog signal, compare the analogsignal with a triangular wave to generate a pulsewidth signalcorresponding to a value of a pixel, and use the generated signal todrive a laser device. In this embodiment, PWM is realized by employingdigital control instead of the analog control.

[0040]FIG. 1 is a block diagram of a main unit realizing PWM accordingto this embodiment and a timing chart thereof. Note that the embodimentdescribes an example in which an input pixel value has 4 bits, i.e., animage can be reproduced in 0 to 15 tones (16 tones). However, the numberof bits of an input pixel is not limited to this example.

[0041] Referring to FIG. 1, reference numeral 1001 denotes a digitaldata output unit storing 32-bit light emission patterns for addresses 0to 15. The digital data output unit is configured with a non-volatilememory (e.g., EEPROM, flash memory or the like) to enable rewritingcontents thereof and maintaining the data when the power is turned off.Reference numeral 1002 denotes input data (4-bit pixel data), which issupplied to the digital data output unit 1001 as an address. Referencenumeral 1003 denotes a control signal. Normally, a read signal isinputted to the digital data output unit 1001 in synchronization withpixel data. Note in a case of rewriting data in the digital data outputunit 1001, the control signal is changed to a rewrite signal, a pixelvalue is supplied as an address, and 32-bit data is supplied through adata bus shown in the drawing, thereby rewriting the data stored in thedigital data output unit 1001. Reference numeral 1004 denotes a shiftregister, which loads 32 bits of data in the position designated by theinput data, and outputs a logical high signal for one clock cycle to thebits having “1”, while shifting the data in units of 1 bit insynchronization with a reference clock 1005 (a 32-fold frequency of theclock of the input data). In other words, for a pattern serially having“1”, a PWM signal 1006 having a width corresponding to the number of 1is outputted. Reference numeral 1007 denotes a pulsewidth additioncircuit which adds a pulsewidth signal for fine adjustment to the end ofthe PWM signal 1006 (details will be described later in the descriptionof pulsewidth addition amount setting processing). The additionalpulsewidth signal can be changed by the control signal 1008 (in thetiming chart shown in the lower portion of FIG. 1, the pulsewidth isincreased by the hatched portion).

[0042] Described next is FIG. 2, showing a block diagram (light amountdetector) of a driving unit of the laser device and its surroundings.

[0043] Referring to FIG. 2, reference numeral 1201 denotes a powersupply for biasing a laser device and a photo-detector (PD). Referencenumeral 1202 denotes an amplifier which amplifies the amount of laserbeam detected by the PD, located inside the laser device. Referencenumeral 1203 denotes a filter for damping high-frequency ripples ornoise components of an output of the amplifier. Reference numeral 1204denotes an averaging processor for obtaining an average value of thefilter's outputs. Reference numeral 1205 denotes an engine of an imageforming apparatus. Reference numeral 1206 denotes an APC light amount FB(feedback) that is a feedback signal for feedback-controlling the amountof laser beam to a predetermined value. Reference numeral 1207 denotesan LD light emission control signal for controlling emission/nonemission of the laser device. Reference numeral 1208 denotes an APCtiming signal for designating opening or closing a control loop of thefeedback control of the amount of laser beam. Each of these controlsignals is supplied to the LD driver 24.

[0044]FIG. 3 is an operation timing chart in a test mode. In FIG. 3,reference numeral 1210 denotes an APC timing; 1211, a beam detecttiming; and 1212, an image zone timing. In this embodiment, assume thata laser beam is emitted when a logical signal is High (hereinafterreferred to as H), and turned off when a logical signal is Low(hereinafter referred to as L). In normal printing processing, a signalcorresponding to each pixel data is outputted in the image zone timing1212. When a test mode is designated from an operation panel (notshown), signals indicative of input data 0 to 15 (although the drawingshows skipped values, in reality, data for 0, 1, 2, . . . , and 15) areserially outputted (for instance, one scan line). Since the test mode isintended to perform detection and correction of the amount of light,conveyance of printing paper or generation of a toner image by adeveloper is not performed.

[0045] Reference numeral 1214 denotes an APC timing where the feedbackloop is closed at H (feedback control is performed) and opened at L.

[0046] Reference numeral 1215 denotes input data; 1216, one pixel cyclein the image writing zone; 1217, a light amount detection valueindicative of an output of the filter; and 1218, an average amount oflight value indicative of the average output value.

[0047] Reference numeral 1209 denotes a signal BD, representingdetection of a laser beam passing detected by a sensor, which is locatedoutside the image writing area by a predetermined width. Referencenumeral 1213 denotes a beam-detected timing.

[0048] Hereinafter, the sequence of the test mode is described withreference to FIG. 3. First, the LD light emission control signal is madeH to emit a laser beam (timing 1210). Then, the APC timing signal ismade H to close the laser light amount feedback loop, thereby generatinga controllable state (timing 1214). In this stage, the APC light amountFB signal corresponding to the filter output is sampled by the LD driver24, and the amount of laser beam is recognized. The amount of sampledlaser beam is compared with a predetermined target value (not shown). Ifthe amount is lower than the target value, the laser driving current isincreased, whereas if the amount is higher than the target value, thelaser driving current is decreased. In this manner, the amount of laserbeam is controlled to match the target value after a predetermineddamping time.

[0049] After a predetermined damping time, the APC timing signal isreturned to L. When the LD driver 24 recognizes this state, it opens(cut off) the feedback loop. At the same time, the control state ismaintained as it is, and the value in this state is fixed for laserdriving.

[0050] Next, normal beam detect operation (treated as horizontalsynchronization signal) is described. Upon adjusting the amount of lightbeam to a predetermined level in the above-described APC operation, theLD light emission control signal is again made H (timing 1211) to turnon the laser at the timing which is appropriately prior to the timingthe laser beam passes the beam detect sensor. When the laser beam passesthe beam detect sensor, the beam detect signal is inverted, confirmingbeam detection that notifies passing of the laser beam (timing 1213).Then, the LD light emission control signal is returned to L to turn offthe laser.

[0051] Next, a pulsewidth addition function (determining processing ofthe signal 1008 supplied to the pulsewidth addition circuit 1007 inFIG. 1) is described.

[0052] Due to a light emission rising characteristic of a laser deviceand a current rising characteristic of a laser driver provided beforethe laser device, a laser beam is not generally emitted by a pulsesignal having a smaller pulsewidth than a predetermined pulsewidth. Forthis reason, for an input data value corresponding to the lowest lightemission (normally 0), a pulse adding up the delay time (the smallestpulsewidth that can emit laser beam) is added to a PWM output from theshift register. Although this pulsewidth addition depends upon theconstruction of the apparatus, in this embodiment, assume that the pulseis added uniformly to each final edge of the pulse 1006 (FIG. 1)regardless of input data as mentioned above (this will be describedlater with reference to FIG. 6).

[0053] Next, a description is provided on the characteristic point ofthe present embodiment, in which the number of binary-valued dots of alight emission pattern for one pixel is made larger (to be morespecific, the twice as many number or more is preferable) than thenumber of types of multivalued input pixel data (4 bits=2⁴=16 types).

[0054]FIGS. 4A and 4B are tables where output driving patterns are setfor each input data. Two tables are provided for appropriatelycorrecting unevenness of laser beam emission devices A and B.

[0055] Since it is assumed in this embodiment that input data for onepixel is 4 bits, there are input data values 0 to 15, and an outputpattern has 32 bits. In FIGS. 4A and 4B, “the number of ON bits”indicates the number of dots (left-aligned) stored in the digital dataoutput unit 1001 in FIG. 1. The hatched portions indicate portions addedby the pulsewidth addition circuit 1007.

[0056] Hereinafter, the process of generating the correction data isdescribed.

[0057]FIG. 5 is a graph showing an average amount of laser beam for aduty of a pulse signal inputted to laser driving devices A and B. In thehorizontal axis, 0 indicates the minimum value 0%, and 255 indicates themaximum value 100%. In the vertical axis, 0 indicates the minimum valueand 255 indicates the APC light amount. The lines 50 and 51 in FIG. 5show bare characteristics of the devices A and B before theaforementioned pulsewidth addition is performed.

[0058] Generally it is assumed that a driving system supplies apredetermined bias current to a laser device even when a minimum-levelpulsewidth for obtaining a laser beam is inputted. Therefore, apredetermined offset LED emission component exists. On the contrary, theaverage amount of light becomes equal to the APC light amount when thepulsewidth is in the maximum level, because of a delay characteristiccaused when the laser device and laser driver are turned off, contraryto the minimum-level pulsewidth. In this embodiment, a description isprovided assuming that the minimum amount of light is 30.

[0059] When input data is 0, in order to perform pulsewidth addition toachieve the amount of light 30 in the devices A and B, it is necessaryto enlarge PWM for input data 0. In this embodiment, a pulsewidth isadded so that the device A (having the bare characteristic 50) achievesthe characteristic 52 and the device B (having the bare characteristic51) achieves the characteristic 53 for input data 0 as shown in FIG. 5(the graph lines practically shift in parallel).

[0060] A method of pulsewidth addition amount setting (the amount ofshifting) is described with reference to the flowchart shown in FIG. 6.

[0061] Note although this processing is performed in an adjusting stepof apparatus assembling, it may be regularly performed by a user.

[0062] In step S1, the number of dots of the light emission pattern fordata input 0 is set to 1 dot (a pattern having “1” followed bythirty-one “0”), thereby setting the pulsewidth addition to 0. In thisexample, the pulsewidth addition value includes a rough-adjustment setvalue and a fine-adjustment set value. Both values are set in 0. In thisstate, input data 0 is serially inputted (step S2). The light amountdetector (1204 in FIG. 2) measures an average value of the amount oflaser beam emission in this stage.

[0063] In steps S3 to 5, the rough-adjustment set value (value set inthe pulsewidth addition circuit 1007 in FIG. 1) is incremented by onestep, and each time the value is incremented, it is determined whetheror not the amount of laser beam exceeds the target value. In thisexample, the target value is 30. When the amount of laser beam exceeds30, the repeated routine ends. When it is determined that the amount oflaser beam exceeds 30, the rough-adjustment set value is returned to thevalue of the previous step in step S6.

[0064] In steps S7 to S9, the fine-adjustment set value is incrementedby one step, and each time the value is incremented, it is determinedwhether or not the amount of laser beam exceeds the target value 30.When the amount of laser beam exceeds 30, the adjustment routine ends.Although it is possible to perform a fine-adjustment setting step thatmore closely achieves the target value by using the value obtained whenthe amount of laser beam exceeds the target value and the value of theprevious step, this example omits such process.

[0065] In the foregoing manner, a pulsewidth that can most closelyachieve the amount of light beam 30 for input data 0 is determined.

[0066] Since the pulsewidth addition is uniformly performed on eachlight emission pattern of all the input data, the relation between theamount of light beam emission and the duty of a pulse signal prior topulsewidth addition is shifted in parallel, as represented by the lines52 and 53 in FIG. 5.

[0067] In this embodiment, since one output pattern is formed with 32dots, pulsewidth adjustment is performed so that the amount of lightbeam 30 is achieved at a position of X coordinate 8 (=256/32) whichcorresponds to 1 dot. Although there may be an adjustment remainderdepending on a resolution of the adjustment steps, the drawing shows thestate of perfect adjustment for the purpose of simplicity.

[0068] Next, a relation between input data and the amount of outputlight is described.

[0069] Conventionally, since the number of dots of the pattern is 2^(n)while the number of bits of input data is n, the amount of output lightis determined on a one-to-one basis with the input data. For instance,in a case where 16 types of 4-bit data are inputted, the number of dotsin a pattern is 16. Therefore, the dot numbers 1 to 16 or 0 to 15 arepractically allocated to the 16 types of input data. On the contrary,according to the present embodiment, it is possible to set the number ofdots twice as many (or more) as the number of types of input data asshown in FIG. 1. Therefore, finer setting is possible. Morespecifically, since 32 dots can be allocated to 16 types of input data,it is possible to make adjustment so that a predetermined relationbetween the input data and the amount of light is achieved.

[0070] For instance, assume that it is to achieve the amount of light 50when input data is 1, achieve the amount of light 200 when input data is14, and achieve the amount of light 255 when input data is 15.

[0071] In FIG. 5, on the point of the step of horizontal axis 8(=256/32), a point that can most closely achieve each target amount oflight is determined. There are cases that plural steps achieving theamount of light 255 exist. In this case, for instance, a rule adoptingthe smallest step is employed. Furthermore, the amount of light 50 to200 are uniformly divided by 13, and the point that most closelyachieves each of the 12 values of the amount of light on the point ofthe step of horizontal axis 8 is allocated to each of the input data 2to 13. Each step determined in the foregoing manner is allocated to thelight emission pattern table as the number of dots in units of 8 steps.

[0072] The obtained number of dots is left aligned and set in the lightemission pattern table as shown in FIGS. 4A and 4B. “The number of ONbits” in the drawing indicates the number of bits of “1” in the datastored in the digital data output unit 1001 in FIG. 1. The hatchedportions correspond to the set contents of the pulsewidth additioncircuit 1007.

[0073] As described above, by virtue of having the number ofbinary-valued dots of a light emission pattern for one pixel twice asmany as the number of types of multivalued input image data, in thelight emission modulation means having in storage means a plurality oftypes of a light emission pattern for one pixel, expressed in anemission/non emission binary-valued dot string, for each multivaluedinput image data, which reads out the light emission pattern from thestorage means in accordance with multivalued input image data inputtedin time series and outputs a light emission driving signal to a lightemission device, uneven transmission characteristics of a laser drivingdevice or laser are absorbed, and the relation between the input dataand the amount of output light can individually be adjusted to apredetermined characteristic.

[0074] Hereinafter, a description is provided on setting processing ofthe number of dots for input data 0 to 15 (to be more specific, inputdata 1 to 15 excluding 0, since the number of dots for input data 0 hasbeen determined in the pulsewidth addition processing).

[0075] To simplify the description, the target amount of light (average)for each input data is set as that shown in FIG. 7, reflecting the abovedescription. In FIG. 7, values of the target amount of light for inputdata 0 to 15 are the final target values.

[0076] In this embodiment, data 0 is serially outputted in the test modeat timing 1212 in FIG. 3 to determine a pulsewidth to be added forachieving the amount of light 30. As a result, a parameter to be set inthe pulsewidth addition circuit 1007 is determined. Thereafter,processing for determining a 32-bit pattern for each of the data 1, 2, .. . , and 15 is performed.

[0077] Herein, an initial number of dots for data 1 is determined(determined similarly for data 2 to 15), and a pattern that most closelyachieves the target amount of light is decided as a dot pattern for data1, and the pattern is stored in the digital data output unit 1001.Hereinafter, details are described.

[0078]FIG. 8 shows processing contents for the input data 1 to 14.

[0079] In step S11, 1 is set in the data address S. This is equivalentto setting input data 1 as an address of the digital data output unit1001 in FIG. 1.

[0080] Next, data having 1 for the first one bit and 0 for the remainingthirty-one bits (if the data is expressed in a hexadecimal numeral (H)with the first bit being a MSB, 80000000H) is set in the address S.

[0081] In step S13, LD driving is performed by serially supplyingdata 1. The driving result corresponds to the result of pulsewidthaddition set prior to this processing. Next, in step S14, the averageamount of light is acquired.

[0082] In step S15, it is determined whether or not the amount of lightis larger than the target amount of light. If the amount of light hasnot reached the target amount of light, a difference (absolute value)between the acquired amount of light and the target amount of light iscalculated in step S16. In step S17, the number of bits of “1” of thepattern stored in the digital data output unit 1001 is increased by 1(increased so that bit 1 is successive), and the control returns to stepS13.

[0083] When it is determined that the acquired amount of light exceedsthe target amount of light, a difference (absolute value) between theacquired amount of light and the target amount of light is calculated instep S18, and the difference is compared with an immediately precedingdifference (the timing at which the maximum amount of light that doesnot achieve the target amount of light is detected). The value having asmaller difference between the acquired amount of light and the targetamount of light is selected and stored in the digital data output unit1001 (steps S19 and S20).

[0084] In step S21, it is determined whether or not the value of thedata of interest is 14. If NO, the control proceeds to step S23. Thenext address position is set in the address S, and a previouslydetermined pattern is set as an initial pattern, in preparation for thepattern determination processing of the next data. Thereafter, theabove-described processing is repeated.

[0085] Next, setting processing of table address S=15 is described withreference to FIG. 9.

[0086] In step S31, the table address S is set in 15 that is the lastdata. One dot is added to the pattern determined for data 14, and thispattern is set as the initial data of this table address.

[0087] In step S32, the laser is driven with the set pattern, and theamount of light is acquired in step S33. If it is determined in step S34that the acquired amount of light does not exceed the target amount oflight (L′(15)), the number of bits of “1” in the pattern is increasedand the processing is repeated from step S32. When it is determined thatthe acquired amount of light achieves the target amount of light, thisprocessing ends and the pattern data is adopted.

[0088] As a result of the foregoing processing, the number of ON bitsfor each input data, which is appropriate for each of the devices A andB, is determined as shown in FIG. 7.

[0089]FIG. 10 is a graph in which the amounts of light achieved by the32-bit patterns for the devices A and B and the added pulsewidth areplotted on the target-value line. By virtue of the above-describedprocessing, the amounts of light emitted by the devices having differentcharacteristics are about the same as the target value as shown in FIG.10.

[0090] As has been described above, according to the present embodiment,it is possible to correct unevenness and variations of the relationbetween input data and the amount of output light, due to a disturbanceincluding not only the variations in initial characteristics of thelight emission device and light emission driving device but alsovariations caused by environmental changes or variations generated overtime. Thus, it is possible to maintain high quality in formed images.

[0091] Note although the above embodiment has described processingperformed in factories, this processing may be executed when apredetermined operation is performed on an operation panel which isnormally included in a printer.

[0092] Furthermore, although the above embodiment has described thatinput multivalued pixel data has 4 bits, the present invention is notlimited to this number of bits.

[0093] Moreover, although the above embodiment has shown an example ofscanning the drum with one laser beam, two laser beams may be generatedas shown in FIG. 14, and the construction according to the aboveembodiment may be provided for each laser beam. In this case, it shouldbe easy to understand if assumed that the two devices a and b in FIG. 14correspond to the aforementioned devices A and B.

[0094] For the components in FIG. 14 which are similar to those in FIG.11, the same reference numerals are assigned to simplify thedescription.

[0095] A two-beam-type laser chip 23, having laser diodes a and b,comprises a photodiode c which receives backlight of each laser diode.The LD driver 24 supplies driving currents Id1 and Id2 for controllingemission of each laser diode. The monitor current Im, detecting theamount of light emitted from the photodiode, is inputted to the LDdriver 24 to perform automatic power control (APC) of the amount oflight emission of the laser diodes a and b. In the laser chip 23, twolaser beams cannot be emitted at intervals of one pixel (about 42 μm for600 dpi) because of the characteristic of the laser chip. For thisreason, the laser chip is arranged at an angle so as to generate twobeams in the positions apart from each other by, for instance, 16 pixels(printing pixel) in the laser scanning direction as shown in FIG. 14. Amodulated laser beam generated by the laser chip 23 is polarized by thepolygon mirror 18, which is fixed to a motor axle and rotated in thedirection of the arrow shown in FIG. 14, and scanned on thephotosensitive drum 20. The f-θ lens 19 is provided to condense thepolarized modulated laser beam on the photosensitive drum 20 at aregular linear velocity. The photosensitive drum 20 and printing tonerare electrostatically charged to predetermined levels in advance. Sincethe amount of toner attachment changes in accordance with the amount oflight irradiated on the photosensitive drum 20, a halftone image can beprinted. The BD mirror 21 is provided with a fixed mechanical positionrelation with the photosensitive drum 20. A laser beam reflected by theBD mirror 21 is inputted to the photoreceptive diode 22, and used todetect a data write starting position on the photosensitive drum 20. Anoutput of the photoreceptive diode 22 is inputted to the horizontalsynchronization signal generator 27 to generate a horizontalsynchronization signal BD. The signal BD is inputted to the pixelmodulator 25. The pixel modulator 25 generates a pixel clocksynchronizing with the horizontal synchronization signal BD or acoefficient-fold clock of the signal BD. Based on the pixel clock, readclocks RK1 and RK2 for reading pixel data are inputted to the pixel datagenerator 26. The pixel data generator 26 outputs pixel data D1 and D2and write clocks WK1 and WK2 to the pixel modulator 25. Based on theinputted pixel data, pixel modulation signals ON1 and ON2, which enablemodulation of a desired amount of laser beam, are outputted to the LDdriver 24.

[0096] To print a color image with the above construction, the two-beamstructure is provided to each of the four drums for Y, M, C and K.

[0097] By employing the aforementioned construction of the aboveembodiment in the printer engine having foregoing construction, it ispossible to realize position deviation correction in an image caused bya timing error of the signal BD in the image writing unit. This iselectrically realized in a level of {fraction (1/32)} pixel bycontrolling a phase (delay) of the pixel clock with absolute pixelposition setting data RG in the pixel modulator 25.

[0098] Since the two-beam laser chip 23 is arranged at a gentle angle asmentioned above, an error or variation of the arrangement angle causesvariations in the beam intervals, which then necessitate pixel positioncorrection. However, the correction is also electrically possible in alevel of {fraction (1/32)} pixel by controlling a phase (delay) of thepixel clock with relative pixel position setting data RP in the pixelmodulator 25.

[0099] Furthermore, it is necessary to correct an error of an imagesize, caused by unevenness in optical machine accuracy of the laser chip23, polygon mirror 18, f-θ lens 19, and photosensitive drum 20. Thecorrection is realized by incorporating a frequency synthesizer in thepixel modulator to change the pixel clock frequency, and then usingpixel frequency setting data DF. Therefore, pixel position setting dataDS for pixel positioning is inputted to the pixel modulator of the imagewriting unit in FIG. 14.

[0100] Although the above embodiment has described an example of aprinter in which a laser beam scans the photosensitive drum, the presentinvention is not limited to this example. For instance, the presentinvention may be applied to an apparatus employing an array of LEDs,each of which is turned on/off for forming an electrostatic latent imageon the photosensitive drum.

[0101] As has been described above, according to the present invention,in a case of performing pulsewidth modulation with the use of a digitaldata pattern, it is possible to perform appropriate pulsewidthmodulation in accordance with variations of a device or a drivingsystem.

[0102] As many apparently widely different embodiments of the presentinvention can be made without departing from the spirit and scopethereof, it is to be understood that the invention is not limited to thespecific embodiments thereof except as defined in the claims.

What is claimed is:
 1. A pulsewidth modulation apparatus for inputtingmultivalued pixel data expressed in a plurality of bits, generating asignal having a pulsewidth corresponding to a value of the multivaluedpixel data, and outputting the generated signal to a predetermined printengine, comprising: storage means arranged to store a digital pattern inan address position corresponding to the value of each multivalued pixeldata in order to generate a binary-valued pattern corresponding to thevalue of the multivalued pixel data; and output means arranged to accesssaid storage means by using inputted multivalued pixel data as anaddress to read a corresponding binary-valued pattern, and output apulsewidth signal corresponding to a state of bits of the pattern,wherein the pattern stored in said storage means has a number of bitslarger than a number that can be expressed by a number of bits of theinputted multivalued pixel data, and the pattern is rewritable.
 2. Thepulsewidth modulation apparatus according to claim 1, wherein the numberof bits of each pattern stored in said storage means is twice as many asthe number that can be expressed by a number of bits of the inputtedmultivalued pixel data.
 3. The pulsewidth modulation apparatus accordingto claim 1, further comprising rewrite means arranged to rewrite thepattern stored in said storage means in accordance with a characteristicof the print engine.
 4. The pulsewidth modulation apparatus according toclaim 3, wherein the print engine includes laser driving means whichperforms emission/non emission of a laser beam in accordance with apulsewidth signal, and light emission amount detection means whichdetects an average value of the emission amount of the laser beam,wherein upon sequentially outputting multivalued pixel test data, anaverage light emission amount is detected for each test data by saidlight emission amount detection means, a pattern to be written isdetermined based on the detected average light emission amount and atarget light emission amount, and the determined pattern is stored insaid storage means.
 5. The pulsewidth modulation apparatus according toclaim 1, further comprising addition means arranged to generate apulsewidth signal, having a pulsewidth smaller than a pulsewidthreproduced by bits of the pattern, and add the generated pulsewidthsignal to the pulsewidth signal outputted by said output means.
 6. Thepulsewidth modulation apparatus according to claim 5, wherein saidaddition means adds a pulsewidth which achieves a lowest density thatcan be reproduced by the print engine.
 7. The pulsewidth modulationapparatus according to claim 1, wherein the print engine is a laser beamprinter engine.
 8. The pulsewidth modulation apparatus according toclaim 7, comprising means for scanning two laser beams on onephotosensitive drum of the laser beam printer engine, wherein each ofsaid scanning means includes said storage means and said output means.9. An image printing apparatus for printing an image on a printingmedium by inputting multivalued pixel data expressed in a plurality ofbits, generating a pulsewidth modulation signal having a pulsewidthcorresponding to a value of the multivalued pixel data, and outputtingthe pulsewidth modulation signal to a print engine, comprising: storagemeans arranged to store a digital pattern in an address positioncorresponding to the value of each multivalued pixel data in order togenerate a binary-valued pattern corresponding to the value of themultivalued pixel data; and output means arranged to access said storagemeans by using inputted multivalued pixel data as an address to read acorresponding binary-valued pattern, and output a pulsewidth signalcorresponding to a state of bits of the pattern, wherein the patternstored in said storage means has a number of bits larger than a numberthat can be expressed by a number of bits of the inputted multivaluedpixel data, and the pattern is rewritable.
 10. A control method of animage printing apparatus for printing an image on a printing medium byoutputting a pulsewidth modulation signal to a print engine, said imageprinting apparatus including: input means which inputs multivalued pixeldata expressed in a plurality of bits; storage means which stores for avalue of each multivalued pixel data a binary-valued pattern constructedwith a number of bits larger than a maximum value that can be expressedby inputted multivalued pixel data; and output means which accesses thestorage means by using the inputted multivalued pixel data as an addressto read a corresponding binary-valued pattern and outputs a pulsewidthsignal corresponding to a state of bits of the pattern, said controlmethod comprising the steps of: detecting a state of the print enginefor each test data by sequentially outputting multivalued pixel testdata; determining a pattern to be written based on the state detected insaid detecting step and a target state; and storing the determinedpattern in the storage means.
 11. The control method of an imageprinting apparatus according to claim 10, said print engine includinglaser driving means which performs emission/non emission of a laser beamin accordance with a pulsewidth signal, and light emission amountdetection means which detects an average value of the emission amount ofthe laser beam, wherein in said detecting step, multivalued pixel testdata is sequentially outputted, and an average light emission amount isdetected for each test data by the light emission amount detectionmeans.
 12. The control method of an image printing apparatus accordingto claim 11, said image printing apparatus including addition meanswhich generates a pulsewidth signal having a pulsewidth smaller than apulsewidth reproduced by bits of the pattern, and adds the generatedpulsewidth signal to the pulsewidth signal outputted by the outputmeans, said control method further comprising the step of setting theaddition means to add a pulsewidth which achieves a lowest density thatcan be reproduced by the print engine.